Problem is in the original mail

The message contains some umlauts encoded correctly ("Vielen Dank für die Wiedervorlage") and some encoded incorrectly ("Sehr geehrter Herr MÃ¼ller"). It appears that the MUA used by the sender of this email is corrupting the umlauts in the quoted text. Zimbra can only display what's in the message, and the quoted part of the message is incorrect.